Fluids are known to form horizontal layers depending on gravity and on the density of the fluid. An example of a property that influences the density of a fluid is temperature. For example, water with the temperature of 4° C. has the highest density of all liquid water. Water stratification occurs when water masses with different properties, such as temperature, form layers. Stratification may be upset by turbulence which leads to mixed layers of water.
A field in which stratification is of importance is the field of energy storages wherein a fluid, such as e.g. water, may be used for storing thermal energy. The thermal energy of the water is preserved to a great extent during an extended period of time, where conduction is the main reason for heat transfer. In such an application, energy may be charged to the storage by retrieving cool water from one level of the storage, heating the retrieved water in a heat exchanger and then returning the heated water to the storage at a level with the corresponding temperature. For discharge of energy hot water is retrieved from the storage, cooled in a heat exchanger and returned to the storage at a level with the corresponding temperature. In this type of application it is important to keep water with different temperatures unmixed when water is supplied to the storage, i.e. to keep the stratification intact. It is thus of importance to create as low turbulence as possible in the energy storage when providing water to the storage.